The field of the invention is safety circuits and the present invention is particularly concerned with a safety circuit for a window lift driven by an electric motor, especially in an automobile.
Safety circuits are known to the art for automotive window motors. Their purpose is to prevent unintentional wedging of body parts when the window pane is moved by a motor in the closed direction, i.e., upwardly. Such safety circuits cause a reversal of the drive motor when the window pane encounters an impedance in its upward motion. When this happens, the drive motor uses more electric current than in the ordinary closure process. It is known to use this increment of current as a criterion for reversing the direction of rotation of the motor.
However, current consumption by the motor is by no means always constant even during a normal closing process; instead, it strongly depends on external conditions, for instance outside temperature, degree of wear of the window guides, etc. Accordingly, it is not possible to merely use the absolute current level as the criterion for reversing the motor rotation. Hence, it has already been proposed to make use not of the current level but only a sudden rise in current di/dt as the reversal criterion. The basic concept, for example in West German Application 3,303,590, published Aug. 9, 1984, is that regardless of the actual instantaneous current drain in the motor, a deceleration of the pane in its upward motion, for instance by a caught hand, will always entail an incremental change in current. The occurrence of such a positive di/dt therefore is always a sign of jamming, and it is proper to use this positive di/dt as a reversal criterion.
However, a difficulty is encountered with this approach in that at motor start-up, a current surge also takes place which decays after only a few milliseconds down to the ordinary operational current. This current surge obviously may not be interpreted by the circuit as being a reversal criterion because otherwise it would never be possible to close the window. Therefore, it has already been suggested to use a positive di/dt as a reversal criterion only when simultaneously the motor rpm denoted by n exceeds zero. This additional condition is based on the consideration that the initial current is already decaying when the motor starts rotating. In that case, the slope of the initial current is negative.
This circuit, however, suffers from the drawback that when a hand, a head, or the like is already wedged before the pane starts moving, then this body part is first subjected to the full pressure from the pane edge exerted by the start-up current on the wedged body part. It is only after the start-up current has begun decaying that the positive di/dt can cause reversal of the direction of rotation. When the wedging is such that the motor cannot even start, whereby n=0, then the safety circuit does not become operative in the first place and the hand remains fully wedged.
This drawback assumes practical significance most of all when the pane has been opened only to such an extent that it is still just possible to insert one's fingers between the pane edge and the upper window frame. If now the closing key or switch for the window is actuated, the pane presses against the fingers with full force for a moment before the safety circuit responds, if at all.